This is a test page that references a slow-loading stylesheet using the new standard link[rel=preload]. A script is included to enable asynchronous CSS loading in browsers that do not support link[rel=preload], and to enable the stylesheet immediately when it is loaded or cached.

That markup looks like this:

<link rel="preload" href="slow.css.php" as="style" onload="this.onload=null;this.rel='stylesheet'">

Note: The CSS file has a 5 second delay built into its server response time. If it is loaded in a non-blocking manner as desired, you should be able to read this text before the page is styled as white text on green background.